Police said an investigation will be carried out by the Health and Safety Executive and the Environment Agency.

Terra Nitrogen said an explosion led to a fire "involving mixed gasses" but the cause of the incident was unknown.

"The plant has now been safely shut down and an investigation is under way," it said in a statement.

"The company will co-operate with any separate investigations carried out by the regulatory authorities.

"Terra Nitrogen UK would like to apologise to anyone who has been inconvenienced by the need for the precautionary actions and would thank people for their patience and understanding during the incident."

'Loud rumble'

Peter Honeyman, 28, from Middlesbrough, was driving home with four friends after a night out in Hartlepool.

He was level with the plant at the time of the first explosion.

"The first blast almost knocked my vehicle off the road. I pulled up and heard two more explosions," he said.

"The sky lit up with orange and red flames. I could feel a burst of heat and an awful smell filled my throat."

Les Gunn, who lives about four miles away from the plant, told BBC News 24 he was woken by an "absolutely deafening" sound.

"The second one was absolutely huge," he added. "There was a huge ball of flame."

Joanne Twaits from Billingham said she lives opposite the site. "There was a huge gush of fire and an astonishing thud," she told the BBC News website.

"It woke my two young children and shook the house as I looked out of my window."